OTDR is an established technique for analysing the propagation of light in an optical fibre. In the telecommunications industry, the technique is widely used to detect and locate damage to optical fibres. The intensity of light backscattered in an optical fibre as a light pulse travels along the fibre can be detected using a photodetector arranged at the end of the optical fibre into which the light pulse is transmitted. Analysing a signal representative of the detected backscattered light generated by the photodetector over time can allow determination of a spatial distribution of attenuation along the fibre. As attenuation tends to be greater at locations of damage, these locations can be identified from the determined spatial distribution.
It has also been recognised that when the light pulse is coherent and propagates in a monomode optical fibre, the backscattered light interferes to contribute a component to the intensity of light detected by the photodetector. The intensity of this component depends on the strength with which the light is backscattered and the phase of the light when it is backscattered, both of which vary randomly and indeterministically along the length of the fibre. In particular, whilst the phase of the light in the light pulse changes as the light pulse travels along the optical fibre at a relatively predictable and slow rate determined by the inherent refractive index of the fibre, external influences such as changes in temperature and pressure or the presence of acoustic waves, can effectively alter the refractive index of the optical fibre and hence the rate of change of phase of the light in the light pulse as it travels along the fibre. The magnitude of the component of the intensity of light detected by the photodetector due to interference can therefore change as these external influences change. This allows changes in the external influences to be detected from changes in the intensity of light backscattered in the fibre.
Conventionally, signals representing the intensity of light backscattered from successive pulses of light travelling along the fibre have been compared to one another and differences between the signals have been identified as changes in external influences affecting the rate of change of phase of light as it travels along the fibre. However, as any coherently related parts of a given light pulse can potentially interfere to contribute to the intensity of light detected by the photodetector, it can be difficult to resolve the location and magnitude of such changes accurately. For example, any detected change in intensity might be indicative of an external influence affecting the rate of change of phase of light as it travels along the fibre between any coherently related parts of the light pulse. Different external influences in different parts of the light pulse can also contribute different intensity components that combine to give a sum change in intensity at the photodetector. So, the resolution of the system is limited at best to the length of the light pulse. However, the light pulse must have a sufficiently long duration to allow the photodetector to generate a useful signal. Resolution is therefore limited. It is also difficult to determine or control the sensitivity of the apparatus to phase disturbances.
Furthermore, as both the strength of the backscatter and the phase of the light at the point of backscatter vary randomly along the fibre, the magnitude of any change in the intensity of the backscattered light detected by the photodetector is not necessarily proportional to the difference in the rate of change of phase of light propagating in the fibre caused by the external influence or such like. Conventional OTDR techniques for detecting such changes are therefore unable to derive information concerning the magnitude of changes in external influences acting on the optical fibre.
For example, U.S. Pat. No. 5,194,847 describes an intrusion detection system employing OTDR. It recognises that OTDR using coherent light can be used to detect changes in external influences that cause phase perturbations in an optical fibre. However, the document only describes making a straightforward comparison between time distributed intensities of light backscattered from successive light pulses propagating along the fibre and locating changes in external influences at positions of changes to the time distributed intensities. U.S. Pat. No. 5,194,847 does not recognise that the changes to the time distributed intensities between successive pulses do not convey accurate information concerning the magnitude of the changes in the external influences.
The paper “Interferometric Optical Time-Domain Reflectometry For Distributed Optical Fibre Sensing”, Sergey V. Shatalin et al, Applied Optics, Vol 37, No. 24, 20 Aug. 1998 discusses the use of OTDR to detect changes in external influences affecting an optical fibre in detail. However, whilst this paper recognises that it is possible to detect the location of changes in effective refractive index of a fibre reliably, it is specifically noted that the magnitude of the change cannot reliably be detected.
The present invention seeks to overcome these problems.